In the past year, we have made progress in the subject of RNA-cation interactions and how RNA can employ cation selectivity. Our previous in vitro selection, biochemical and structural studies of the bacterial glmS ribozyme-riboswitch had demonstrated that a point mutation can convert the coenzyme-dependent wild-type RNA into a metalloenzyme. In new work, we have subjected the metalloenzyme variant to further in vitro evolution, obtaining a ribozyme with exquisite selectivity for calcium ion over magnesium ion. This RNA is over 10,000 times more active in the former than the latter. Biochemical probing suggests that calcium selectivity was acquired by a pre-existing, formerly non-specific cation binding site which functions primarily in structural stabilization in the wild-typre ribozyme. In the mutant, this site has been co-opted to participate in the transition state when bound to calcium. This is thus a clear case of molecular exaptation.